ALDABRA
PHYSICAL DESCRIPTION OF ALDABRA Aldabra Atoll (9º 24' S, 46º 20' E) lies 400km N of Madagascar and 1100km SW of Mahe, the main island of the granitic Seychelles. It is a large (34km by 14.5km), slightly raised coral platform with a land surface area of approximately 155km² (Stoddart et al. 1971; Taylor et al. 1979). It comprises four large islands – Grande Terre, Malabar, Picard and Polymnie – which form a land rim around a substantial, shallow lagoon (almost 30km across). The lagoon is dotted with numerous smaller islets (up to ca 50 ha) and the atoll is fringed by an intact reef (Stoddart et al. 1971; Taylor et al. 1979). The lagoon is strongly tidal, experiencing two tides daily and a spring-tide maximum range of ca 2.7m (Farrow & Brander, 1971). Aldabra is part of an archipelago, referred to as the Aldabra Group, comprising Aldabra and Cosmoledo atolls and Astove and Assumption islands. Aldabra is the coraline tip of a volcanic seamount, rising from depths of 4000 – 4500m in the SomaliBasin (Stoddart et al. 1971). Subsidence has placed Aldabra's volcanic base will below the present se level, while the raising and lowering of sea-levels has exposed marine (predominantly fossilised coral) limestone surface rocks (Stoddart et al. 1971). Fryer (1911) described the surface features of Aldabra, introducing the locals terms champignon for the heavily dissected, deeply pitted and solution-fretted rock, and platin for the extensive areas of relatively flat, smooth rock. This dichotomous classification is an oversimplification, but is useful nonetheless (Stoddart et al. 1971). Champignon is almost certainly derived from the Creole term for the mushroom-shaped islets that typically consist of the pitted rock and are so abundant on Aldabra. Aldabra was completely submerged during the last interglacial, around 140-120ka B.P. (ka: 1000 years, B.P.: Before Present), when the sea surface around Aldabra was ca 10m above its present level (Thomson & Walton 1972). It re emerged around 100 ka B.P. and reached an estimated maximum height of around 120m a.s.l. during the Wisconsin glacial maxim ca 17 ka B.P. (Taylor et al. 1979). At this point it land area measured ca 400km², more than double the present area. A subsequent rise in sea level resulted in the breaching of the land aim in four places, and consequent filling of the lagoon, around 5 ka B.P.. There has been no substantial geological change since then, with the exception of the widening and deep incising of existing channels between the lagoon and open sea. The current maximum land-elevation (excluding dunes) is 8m a.s.l., at Ile Esprit (Stoddart et al. 1971). HISTORICAL BACKGROUND Human impacts on Aldabra
Aldabra was certainly visited by pre-European seafarers, although they left little visible evidence (Stoddart 1971a). Aldabra appears, under various pseudonyms, on Portuguese charts from as early as 1517 (Stoddart 1971a). Eraly European voyagers occasionally visited Aldabra, starting with the Charles and Elisabeth in 1742, but its situation away from major shipping lanes combined with lack of fresh water discouraged settlement (Stoddart 1971a). It is more recent human history is one of exploitation and very near biological disasters. The establishment of a settlement in the granitic Seychelles in the 18th Century saw the advent of frequent, artisanal fishing trips to Aldabra (Stoddart 1971a). These sorties made extensive use of marine turtles, tortoises and probably other land animals, for food. They may also have been responsible for the introduction of the Black (or Ship) Rat Rattus rattus, and the Domestic Cat Felis catus. The leasing of Aldabra for exploitation of its natural resources began in 1888, while under the aegis of Britain's Colonial Mauritius administration. The decision to tender a commercial lease created considerable public outcry. This included a letter to the colonial administration from eminent biologists of the day, including Charles Darwin, expressing particular concern over the conservation of the last extant population of the region's giant tortoises, the Aldabra Giant Tortoise Dipsochelys dussumieri (Stoddart 1971a). A lease was granted without legislated protection for the tortoises (Stoddart 1971a). Attempts were made to cultivate Cotton Ricius communis, Sisal Agave sisalana and Coconuts Cocos nucifera, with only the latter proving marginally viable (Stoddart 1971a). At the start of the 20th Century, plans were made to mine phosphates, a fate that befell all of the other islands of the Aldabra Group. Fortunately, Aldabra's coral limestone contains only low-grade phosphate deposits (Skerret & Mole 1995); had this not been the case Aldabra could well have become as ecologically devastated as nearby AssumptionIsland. The British Government tool a sinister interest in Aldabra early in the 1960's embarking on a covert operation to assess Aldabra's suitability as a military base. British transferred Aldabra into the British Indian Ocean Territory (B.I.O.T.) in 1965, preparatory to building the base (Stoddart 1971a). Public outrage, spearheaded by the British Royal Society, resulted in a campaign that became known as "the Aldabra Affair" (Skerret & mole 1995). The military agenda was eventually dropped, but the Royal Society maintained a strong interest in Aldabra. Early in the 1970s a research station was built and the atoll's lease was formally handed over to the Royal Society in 1976. At the same time Aldabra was returned from the B.I.O.T. to the Seychelles, which gained independence in that year. In 1979 a Presidential Decree created a statutory body, the Seychelles Island Foundation (SIF), to administer Aldabra (Beaver & Gerlach 1998). The Royal Society donated all the buildings and equipment of the research station to SIF in 1980 (D. Stoddart pers. Comm..). In 1981 Aldabra was designated a Special Reserve under Seychelles law (Order S/I 86/1981) and was declared a UNESCO World Heritage Site in 1982, in recognition of its outstanding natural beauty and to conserve it in perpetuity (Skerret & Mole 1995). FLORA AND FAUNA Habitat types Aldabra's vegetation is a unique example of raised coral vegetation that remains largely free of human transformation, and is strongly influenced by a reptilian herbivore, the Giant Tortoise (Gibson & Phillipson 1983). Casuarina equistefolia groves on the seaward rim, and stands of mangroves (Rhizophora sp., Bruguieria sp. and Ceriops sp.) fringing the lagoon are, with few exceptions, the only trees – the rest of the atoll is a remarkably uniform landscape with vegetation height seldom exceeding four metres. Where the land is close to saline groundwater, vegetation is dominated by the shrub Pemphis acidula, which usually grows in dense, monotypic canopy stands with a low diversity of understorey plants. Other researchers (e.g. Huxley 1982) have found it useful to divide mixed scrub vegetation (i.e. non-Pemphis dominated) dichotomously into "open" mixed-scrub and "dense" mixed-scrub, the former characterised by large areas of bare rock or grass cover, the latter being mostly closed-canopy. Of the 176 flowering plants native to Aldabra, ca 40 are endemic to either Aldabra or to the Aldabra Group. The scale and topographical heterogeneity of Aldabra have given rise to a relatively high diversity of plant communities and assemblages for a coral atoll (Gibson & Phillipson 1983). Yet, despite this species diversity and complexity of vegetation patterns, all extant endemic landbirds have been observed in all habitat types, differing only in their usage of habitat types and in relative abundances around the atoll (R. Wanless unpubl. data). Reptiles The Giant Tortoise is perhaps Aldabra's most renowned resident, being the last representative of a taxon that was once spread across the entire region (Coe & Swingland 1984). Subsistence and commercial exploitation is believed to have rendered all other giant tortoise species in the region extinct by the mid 19th Century (Coe & Swingland 1984), and very nearly exterminated the Aldabran population (Stoddart 1971a). It continues to serve as a flagship species for the conservation of the atoll. Two species of marine turtle nest on beaches on Aldabra: the Green Turtle Chelonia mydas and Hawksbill Turtle Eretmochelys imbricata (Frazier 1984). Both were, like the tortoise, heavily exploited and in urgent need of conservation (Frazier 1984), but more recently have benefited from the protection of Aldabra (Mortimer 1988). Other reptiles found on Aldabra are the House Gecko Hemidactylus mercatorius, Bouton's Skink Cryptoblepharus boutonii and an endemic subspecies of Abbott's Day Gecko Phelsuma abbotti abbotti (Cheke 1984). Seabirds Aldabra has ten species of breeding seabirds (Diamond & Penny 1971) and is of both regional and global importance for several of these. It hosts possibly the largest population of Red-tailed Tropicbirds Phaethon rubicauda and the second largest colony of frigatebirds in the world (Fregata ariel and F. minor) (Reville 1983, Stoddart 1984). The Red-footed Booby Sula sula breeds there in greater numbers than anywhere else in the region. Regionally it is important as one of two breeding localities for Black-naped Terns Sterna sumatrana and Caspian Terns Hydroprogne caspia and there are significant numbers of breeding White-tailed Tropicbirds P. lepturus (Stoddart 1984, Diamond & Prýs-Jones 1986, Wanless & White in press). Shorebirds and waterbirds Aldabra is of little significance for the 15 species of migrant shorebirds recorded there, with the exception of the Crab Plover Dromas ardeola, which winters in significant numbers (Feare & Watson 1984, Betts 2000). Five species of the family Ardeidae breed on Aldabra. The Madagascar Squacco Heron is known to breed only on Aldabra and Madagascar (Hancock & Kushlan 1984). Aldabra is the only atoll and one of only two oceanic breeding sites for the Greater Flamingo Phoenicopterus ruber (Benson & Penny 1971, Rainbolt et al. 1997). Landbirds Fifteen species of landbirds (including the sacred ibis Threskiornis aethiopica abbotti in this definition) have bred on Aldabra in historical times (Benson & Penny 1971). The endemic Aldabra Brush Warbler Nesillas aldabranus became extinct between 1983 and 1986 (Hambler et al. 1985, Roberts 1987). The Barn Owl Tyto alba became locally extinct for unknown reasons some time in the 1960s (Benson & Penny 1971). Aldabra has a single remaining endemic bird species, the Aldabra Drongo Dicrurus aldabranus (Benson & Penny 1971). Of the remaining 12 species, only the Pied Crow Corvus albus has not been described as a distinct subspecies. All endemic forms except the Comoro Blue Pigeon Alectroenas sganziniminor have Madagascan affinities (Benson & Penny 1971). The Aldabra Rail is flightless, unlike its close relative from Madagascar, and its taxonomic status is in need of review. It is the last flightless bird of the tropical western Indian Ocean islands. Introduced mammals The Black Rat became established on Aldabra before historical records were kept. It is ubiquitous and abundant throughout most of the atoll. Its semi-arboreal habits mean that both ground-and tree-nesting species are vulnerable to predation (Racey & Nicoll 1984). The subspecies found on Aldabra is frugivorous, a race with a mostly vegetation diet (Racey & Nicoll 1984). Evidence of Rat predation has been found on even quite distant islets in the lagoon, where it depredates eggs and nestlings of seabirds and shorebirds (pers. obs.). The impact of Rats on the ecology of Aldabra has never been quantified, although they are known to impact heavily the nesting success of all the endemic passerine species (Frith 1976, R. Wanless unpubl. data). Control at present is limited to trapping around the station. Feral cats were first recorded on Aldabra in 1890, but may have colonised before then (Abbott 1893). They have recorded form Malabar and Picard islands, but have only been consistently recorded form Grande Terre (Stoddart 1971a, Huxley 1982, Hambler et al. 1993). This pattern has never been properly explained, but is possibly a function of water availability: only the latter island has standing fresh water year-round (Stoddart 1971a) and is thus able to support a population of Cats through the long, dry monsoon. This might explain the current absence of Cats from Picard. They were never explicitly targeted for removal, but nonetheless have not been seen since pet Cats were removed in the 1970s (l. Chong-Seng inlitt.). Goats Capra hircus were brought to Aldabra in 1890 (Stoddart 1971a). By 1971 they had colonised the whole atoll (barring lagoonal islets). An eradication programme removed Goats from all other islands but had not completely eradicated them from Grande Terre when the programme ended in 1997 (M. Bergeson in litt.). Currently, their numbers appear to be increasing (Betts 2000). THE ALDABRA RAIL The Aldabra Rail is a medium-sized rail (tail to bill length 29-33cm, n = 3). It has a slender build, with long, fairly slender neck, bill, legs and feet. Its wings are somewhat reduced and do not protrude beyond the body when at rest. They are seldom used and when closed, they blend with the rest of the plumage to give the impression of lacking wings entirely. The Aldabra Rail's body is also quite slender, an impression that is emphasised by the noticeable reduction of the breast musculature, and thus the overall impression is of a small, but graceful bird. The Aldabra Rail is an inquisitive, curious bird. Adults show little fear of humans and, after initially investigating an approaching human, will generally ignore observers and continue with regular activities (Penny & Diamond 1971, pers. obs). The species' inquisitiveness was exploited to locate birds and their tolerance of human proximity allowed observations of birds to be made with relative ease. This enabled me to document much of their ecology, behaviour and biology, previously unknown or unpublished. Text courtesy of Mr. Ross Mcleod Wanless and Seychelles Island Foundation (S.I.F) | 
